This invention will be described in connection with apparatus used primarily for emission control useful in natural draft exhaust stacks. Heretofore, catalytic converters for natural draft applications have been made of corrugated thin metal strips laminated with flat strips to prevent nesting of the corrugations of one layer from nesting into the corrugations of an adjacent or contiguous layer. The flat on corrugated configuration has the effect of multiplying the cell density by two and thus increases back-pressure. It is a primary purpose of the present invention to provide a monolith with structural integrity at very low cell densities, e.g., less than about 100 cells/square inch. Low back-pressure in converters is essential in natural draft applications. Further, low back pressure reduces the operating cost of forced draft fans in the exhaust stream.
The corrugated thin metal strips of the present invention are readily produced by a process such as the one described in U.S. Pat. No. 4,711,009 date Dec. 8, 1987 to Cornelison et al. Here, a continuous strip of thin metal foil is passed between partially enmeshed corrugating gears to corrugate the foil. In the preferred embodiments, the corrugations are straight-through and normal to the longitudinal marginal edges of the strip. A coating of a refractory metal oxide may be applied by a wash-coating technique such as described in said patent, and a noble metal catalyst applied thereover, also as described therein. After drying and calcining, the strip may be further treated to cut strips of predetermined length, or to bend and accordion fold the strip. In the present case, it is preferred to cut the strips to a predetermined length and omit the remaining steps of the process as described in the aforesaid patent.
The devices of the present invention are preferably not electrically heated, although for certain applications they may be electrically heatable.
While in most cases, ordinary stainless steel may be used to form the corrugated thin metal strips, and the wire frames of the present invention, certain applications of the devices hereof will require a "high" (1500 F. to 2300 F.) temperature resistive, oxidation resistant metal alloy with low creep properties at operating temperatures. One such alloy is "ferritic" stainless steel. Suitable "ferritic" stainless steel alloys are disclosed in U.S. Pat. No. 4,414,023 dated Nov. 8, 1983 to Aggen. A specific ferritic stainless steel alloy contains 20% chromium, 5% aluminum, and from 0.002% to 0.05% of at least one rare earth metal selected from cerium, lanthanum, neodymium, yttrium, and preseodymium, or a mixture of two or more of such rare earth metals, balance iron and trace steel making impurities. A ferritic stainless steel is commercially available from Allegheny Ludlum Steel Co. under the trademark "Alfa IV."
Another metal alloy useful herein is identified as Haynes 214 alloy, which is also commercially available. This alloy and other nickeliferous alloys are described in U.S. Pat. No. 4,671,931 dated Jun. 9, 1987 to Herchenroeder et al. These alloys are characterized by high resistance to oxidation and creep. A specific example contains 75% nickel, 16% chromium, 4.5% aluminum, 3% iron, optionally trace amounts of one or more rare earth metals except yttrium, 0.05% carbon and steel making impurities. Haynes 230 alloy, also useful herein, has a composition containing 22% chromium, 14% tungsten, 2% molybdenum, 0.10% carbon, and a trace amount of lanthanum, balance nickel. The ferritic stainless steels and the Haynes alloys 214 and 230 are examples of high temperature resistive, oxidation or corrosion resistant metal alloys. For most applications, these and other alloys alloys are used as "thin" metal strips from 1 to 12 or more inches wide, and having a thickness of from about 0.001" to about 0.05" and preferably from 0.0015" to 0.003".